Catalytic cracking apparatuses are crucial means for crude oil refining, and the economic benefits of refineries depend on the overall product distribution of these apparatuses, especially the on the yield of high-value light oil products. Recently, because of the growing trend towards heavier crude oils, a higher heavy-oil-conversion capacity and a higher yield of light oil are demanded for FCC catalysts. As a critical active component determining the reaction selectivity of heavy oil cracking catalysts, a type Y molecular sieve with high target product selectivity and high active stability has been the key technical subject of research in the field of catalysis.
To pursue a high-value light oil yield and improve target product selectivity, elements such as magnesium and phosphorous are used both domestically and abroad to modify and adjust the molecular sieve to have suitable acidity, so as to control the ratio between different reactions during the cracking process, reduce further cracking of light oil, and improve the yield of light oil.
Patent CN1683474A describes a preparation method of a co-catalyst that improves the yield of the target product diesel, comprising steps of calcinating kaolin at 900° C. for its in situ crystallization into a type Y zeolite co-catalyst, and subsequently subjecting it to an exchange modification by using salts of magnesium, rare earth and/or ammonium so as to produce a co-catalyst having a high diesel yield. Stability is not described with regard to this method.
Patent CN1217231A describes a preparation method of a phosphorous-containing faujasite catalyst that improves the yield of the target product diesel, comprising steps of thoroughly mixing a faujasite having less than 5% by weight of Na2O, with or without rare earth, and an aqueous solution of a phosphorous-containing compound, then allowing the mixture to stand for 0 to 8 hours, performing drying and calcination to obtain a phosphorous-containing faujasite, and then producing a catalyst with a high diesel yield by a semi-synthetic process. Stability is not described with regard to this method either.
Extensive investigations have been carried out in domestic and abroad research institutions in order to improve the cracking activity and activity stability of type Y molecular sieves. Currently, it is largely agreed that the framework structural stability and the activity stability of molecular sieves can be improved by localizing as many rare earth ions as possible in sodalite cages in the process of rare earth modification of molecular sieves so as to suppress dealumination of the molecular sieve framework during steam aging. Patent ZL200410058089.3 describes a method for preparing rare earth-modified type Y molecular sieves, comprising steps of adjusting the pH of the system to 8-11 using an alkali solution after completion of the rare earth exchange reaction, and then carrying out conventional subsequent treatment processes. In the molecular sieves prepared by this method, rare earth ions are completely located in small cages (sodalite cages). Patent ZL200410058090.6 describes the reaction performance of the molecular sieves of ZL200410058089.3, wherein the catalyst reaction results show that localization of rare earth metals in sodalite cages improves the structural stability and the activity stability of the molecular sieves, manifested in that the heavy oil conversion capacity of the catalyst is greatly improved, although this catalyst has poor coke selectivity.
US patents like U.S. Pat. Nos. 5,340,957 and 4,584,287 describe a method for modifying type Y molecular sieves, comprising steps of modifying the raw material, i.e. a NaY molecular sieve, via an exchange reaction with rare earth and/or Group VIII elements, and then subjecting it to a hydrothermal treatment to afford an ultra-stable rare earth type Y molecular sieve having high stability. Localization of the rare earth ions or grain distribution is not described with regard to the method.
Chinese patent ZL97122039.5 describes a preparation method of ultra-stable Y zeolites, comprising steps of putting a Y zeolite into contact with an acid solution and an ammonium-containing solution, and subjecting them to a high-temperature steam treatment, wherein the amount of the acid used is 1.5 to 6 moles of hydrogen ions per mole of framework aluminum, the concentration of the acid solution is 0.1 to 5 N, the Y zeolite is kept in contact with the acid solution at a temperature of 5 to 100° C. for a duration of 0.5 to 72 h, and the weight ratio between the Y zeolite and the ammonium ion is 2 to 20. The modification method in accordance with this patent requires addition of an ammonium-containing solution for the purpose of lowering the sodium oxide content in the molecular sieve or reducing the damage to the molecular sieve structure caused by acidic gases during calcination. However, this technique has the following technical disadvantages: 1) since a large number of ammonium ions are added in the preparation process, ammonium-containing ions eventually enter the atmosphere or waste water, increasing ammonia nitrogen pollution and the cost for pollution control; 2) the method of this patent is unable to solve the issue of particle agglomeration in molecular sieves, which issue reduces specific surface area and pore volume of the molecular sieve and increases the obstruction in the pore channel during exchange in the molecular sieve, making it difficult to accurately and quantitatively localize the modifying element in the cages of the molecular sieve; 3) moreover, in this patent it is further mentioned that rare earth ions may also be introduced by ion exchange, during or after the contact between the Y zeolite and the ammonium-containing solution, and that during the ion exchange, ammonium ions compete with rare earth ions and preferentially take up the positions intended for rare earth ions, thereby hindering rare earth ions from entering the cages of the molecular sieve by exchange, and also lowering the utilization of rare earth ions.
Chinese patent ZL02103909.7 describes a method for preparing rare earth-containing ultra-stable Y molecular sieves by subjecting a NaY molecular sieve to one exchange process and one calcination process, characterized in that the NaY molecular sieve is placed in an ammonium-containing solution and subjected to chemical dealumination at 25 to 100° C. for 0.5 to 5 h, wherein the chemical dealumination chelating agent contains oxalic acid and/or oxalate salts, a rare earth solution is then introduced under stirring to produce a rare earth precipitate that contains rare earth oxalate, and the precipitate is filtered and washed to give a filter cake, followed by a hydrothermal treatment to afford the molecular sieve product. Although the molecular sieve prepared by this method has certain resistance to vanadium contamination, it has relatively low activity stability and cracking activity, and is insufficient to meet the requirement set out by the growing trend towards crude oils having higher density and poorer quality. This issue is mainly attributed to the distribution of rare earth ions in the super-cages and sodalite cages of the molecular sieve during modification. This method demonstrates that rare earth ions are present in the molecular sieve system in two forms, i.e., a part of the rare earth enters sodalite cages in an ionic form, while the other part is scattered over the surface of the molecular sieve as an independent phase of rare earth oxide (the precursor of which is rare earth oxalate and is converted into rare earth oxide after subsequent calcination). Such distribution reduces the stabilizing and supporting effect of rare earth ions on the molecular sieve structure. Furthermore, this method also poses a remarkable problem of ammonium nitrogen pollution, and the oxalic acid or oxalate salts added are also toxic and detrimental to the environment and human.
Chinese patent 200510114495.1 describes a method for increasing the rare earth content in ultra-stable type Y zeolites. In this method, an ultra-stable type Y zeolite and an acidic solution at a concentration of 0.01 to 2 N are sufficiently mixed in a solid-to-liquid ratio of 4˜20 at a temperature of 20 to 100° C., treated for 10 to 300 minutes, washed and filtered, then subjected to rare earth ion exchange upon addition of a rare earth salt solution, and then washed, filtered and dried after the exchange, to afford a rare earth ultra-stable type Y zeolite. In this invention, a type Y molecular sieve obtained from water-vapor ultra-stabilization calcination is used as the raw material and subjected to a second exchange and a second calcination for chemical modification, but no investigation on dispersibility of molecular sieve particles is involved.
CN200410029875.0 discloses a preparation method of a rare earth ultra-stable type Y zeolite, characterized in that, in this method, a NaY molecular sieve is subjected to ion exchange with an inorganic ammonium solution first, and then subjected to a water vapor ultra-stabilization treatment to obtain a “one-exchange one-calcination” product; the “one-exchange one calcination” product is then added into a mixed solution of a rare earth salt and citric acid or a mixed solution of an inorganic ammonium salt, a rare earth salt and citric acid, and subjected to an exchange reaction at a certain temperature; and, after completion of the reaction, the molecular sieve slurry is filtered, washed, and eventually calcinated at 450 to 750° C. in air or under a 100% water vapor atmosphere for 0.5 to 4 hours. In this technique, the rare earth modification involves a second exchange modification of the “one-exchange one-calcination” product that serves as the raw material. Because of the lattice contraction in the molecular sieve after the “one-exchange one-calcination”, fragmental aluminum inside the pores clogs the pore channels, increasing the hindrance to rare earth ion exchange, which renders it difficult to accurately localize rare earth ions in sodalite cages.
In order to improve the activity stability of type Y molecular sieves and also improve their selectivity for target products, the present invention employs a precise localization technique for rare earth ions to prepare a type Y molecular sieve with high activity stability and structural stability, which is then modified with magnesium and adjusted to have suitable acidity, so that the ratio between different reactions in the cracking process is under control and the yield of light oil is improved.